Highly efficient compact capacitance coupled plasma reactor/generator and method

ABSTRACT

A compact capacitively coupled electrode structure for use in a gas plasma reactor/generator is disclosed. The electrode structure comprises a parallel plate type anode and cathode spaced to define a gas flow path or volume therebetween. A plurality of electrically conductive fin elements are interposed in the space between the anode and cathode. The fin elements substantially increase the ratio of electrode surface area to volume, and subdivide the gas flow path or volume, thereby substantially increasing the efficiency of plasma gas processing that is possible over a broad range of operating parameters, without substantially increasing the spacing between the anode and cathode. Static or closed operation is also disclosed. Also disclosed is a multi-anode/multi-cathode electrode assembly embodying the basic electrode structure and a highly efficient and compact gas plasma reactor/generator employing the assembly.

RELATED APPLICATIONS

This application is a division and claims the benefit of U.S. patentapplication Ser. No. 09/553,696 filed Apr. 21, 2000 now U.S. Pat. No.6,576,202.

FIELD OF THE INVENTION

The present invention relates to gas ionization apparatus and methodsgenerally and more specifically to capacitance coupled gas plasmareactors and methods.

BACKGROUND OF THE INVENTION

Capacitance coupled plasma reactors are usually constructed with a pairof parallel plate electrodes facing each other, spaced apart inparallel, and placed inside a vacuum chamber. An external electricfield, either DC or AC, is applied to the opposite electrodes. Under lowpressure and with proper spacing between the electrodes, a stable plasmacan be generated and maintained by first ionizing and then creating aglow discharge in gas flowing between the electrodes. Multiple pairs ofalternating polarity parallel plates can be spaced apart and/or stackedtogether to form multiple regions where plasma discharge may occur. Suchcapacitance coupled plasma reactors have been widely used in a varietyof industries for applications such as substrate etching, substratecleaning, substrate film deposition, gas treatment, ion beam source andfor various chemical reactions.

As the term “capacitance coupled plasma” implies, the electrodes form acapacitor, typically of the parallel plate type. The most fundamentaltype is simply two flat plates of opposite electrical polarity and isoften referred to as a “planar diode.” The electrodes may be arranged ina variety of geometric configurations, including configurations havingcurved surfaces, such as concentric parallel cylinders or concentricspheres with parallel tangents. Typically the surfaces of the alternatepolarity electrodes will be equally spaced throughout the structure tomaintain the parallel plate relationship. The geometric regularity andsymmetry between the surfaces of the electrodes in such structures arethought to be desirable for the production of a uniform electric fieldand hence a more uniform plasma. Concave or convex pairs of flat plateelectrodes have also been used to focus or defocus the intensity of theplasma concentration in specific regions for special applications suchas focus sputtering, focus etching or to provide a focused ion source. Anumber of prior art capacitively coupled parallel plate electrodedesigns having different geometric configurations are taught in U.S.Pat. No. 4,735,633, entitled Method and System for Vapor Extraction FromGases, which issued to the inventor of the present invention and isassigned to the assignee of the present invention. The electrodeconfigurations taught in the '633 patent provide large surface area tovolume ratio for compact plasma reactors. Reactors employing electrodeconfigurations as taught in the '633 patent have been successfully usedin industry to provide greater than 99% reaction efficiency.

In addition to electrode spacing, another critical parameter for plasmageneration and maintenance in capacitively coupled plasma reactors isthe operating pressure. A stable glow discharge plasma can be moreefficiently and easily maintained at lower pressures. This is becausethe production and maintenance of the plasma depends on the ionizationof gas molecules in the reactor volume to produce sufficient secondaryelectrons to participate in the cascade collisional ionization processto offset and balance the loss of electrons (and ions) to the electrodesurfaces. The mean free path, i.e., the average distance a primaryelectron will travel in the reactor volume before colliding with amolecule to create secondary electrons, depends upon the operatingpressure. Generally, the higher the pressure, the smaller the value ofthe mean free path. The value of the mean free path places a limitationon the distance in which primary electrons within the electric fieldpotential between the electrodes can accelerate to acquire theionization potential energy needed to facilitate the ionization process.Thus, the smaller the value of the mean free path, the less ionizationpotential energy an electron will acquire for a given operatingpotential before colliding with a gas molecule, and the less secondaryionization is likely to occur.

For a given operating pressure, the electrode spacing determines thenumber of mean free path ionization collisions an electron will beinvolved in before it reaches and is lost to the electrode surface. Forvery short electrode spacing, no glow discharge can be generated andmaintained. This space is known as dark space. Once a plasma is ignitedin the reactor volume, it becomes a conducting sheet itself equivalentto an electrode. Between the plasma and the electrodes, there is alwaysa space gap in which glow discharge ionization does not occur. Only ionsand electrons are accelerated in this gap without further glowionization discharge, and such space is the known as the “dark spaceshield.” The thickness of the dark space shield is also pressuredependent.

Thus, the point at which the gas molecules will break down and a stableglow discharge plasma can be generated and maintained depends on therelationship of the applied external electric field potential, thebreakdown voltage, the electrode spacing and the operating pressure.Paschen experimentally found that the breakdown potential voltage (V)varies with the product of pressure P (in units of Torr) and theelectrode spacing d (in units of cm). The relationships Pashenidentified are known as the law of glow discharge and are reflected inthe “Paschen curves” shown in FIG. 1. FIG. 1 shows Paschen curves 10 forseveral different gases. The electrode design for a capacitivelycoupled, parallel plate plasma reactor must adhere to the physicalrequirements shown by the Paschen curves.

The Paschen curves 10 of FIG. 1 show there is a minimum breakdownvoltage (V) for every gas for the product of Pd at about 1 Torr-cm,i.e., at about point 15. Thus, in practical terms, if the spacingbetween parallel plate electrodes is fixed at about 1 cm, the lowestexternal voltage necessary to apply to the electrodes to initiateionization and breakdown of a gas under vacuum is obtained at a pressureof about 1 Torr. As can be seen from the Paschen curves 10, for a givenelectrode spacing d, as the pressure P increases, the minimum externalvoltage necessary to satisfy the 1 Torr-cm breakdown parameter slowlyincreases. However, as pressure is reduced, the minimum necessaryvoltage sharply increases (in linear scale of Pd). Thus, for example,given a power supply that can provide a maximum voltage of 1000 V, areactor with fixed electrode spacing of about 1 cm can be operated atpressures up to about 300 Torr for neon gas, for example for neon lightapplications. But the same 1000V power supply will not be capable ofgenerating and maintaining a plasma in Neon gas at pressures below about0.1 Torr unless the electrode spacing is increased several times, suchthat the breakdown voltage 15 of the Paschen curve 10 occurs at a Pdvalue below the 1000V maximum supply limit.

Thus, in practical application, the relationships shown in the Paschencurves 10 determine the minimum electrode spacing and hence the minimumsize for a reactor for a given power supply rating and operatingpressure range In most applications, it is desirable to use a lowvoltage power supply, either AC or DC, rather than a high voltage powersupply because of the intrinsically lower cost of lower voltagesupplies. It is also desirable to use smaller spacing between electrodesso that the reactor will be smaller and more compact. However, whenoperating at pressures below about 0.5 Torr, which may be required incertain applications such as in many semiconductor processingapplications, it is a must to increase the electrode spacing to a fewcentimeters or more, thus increasing the reactor size, or alternativelyto employ considerably more expensive high voltage power supplies.Though additional magnetic field sources could be used to confine theplasma in very low pressure operation application, this solution is verycostly, further complicates and upsets the capacitive coupling of theapplied and dissipated plasma energy, and introduces more side effects.

The aforementioned '633 patent teaches to maximize the efficiency of areactor of a given size by maximizing the surface area of the electrodeswithin the reactor volume in a specific way to increase the reactionefficiency. Although the reactor taught in the '633 patent was primarilyintended for use in semiconductor fabrication applications to break downand dispose of noxious exhaust gases, the plasma processing described inthe patent also provides a very efficient means to process materials,such as by sputtering, etching, deposition, surface treatment, etc. Italso provides an efficient gaseous chemical reaction means to producedesirable byproducts, for example chemical synthesis, polymer formation,chemical dissociation, etc. Advantages of this type of plasma processingover other chemical methods include substantially reduced energyconsumption and substantially improved reaction efficiency at relativelylow temperatures. One plasma reactor of the type taught in the '633patent that has been used commercially is trademarked DryScrub® and issold by the assignee of the present invention. As taught in the '633patent, the DryScrub® reactor takes advantage of a large electrodesurface area to plasma volume ratio and a long gas flow path to maximizechemical reaction on the electrode surfaces. This maximizes the reactionrate and reaction efficiency compared to gas phase reaction in the gasstream itself.

Thus, as taught in the '633 patent, for a pair of parallel plateelectrodes, the area of the face of each surface of each electrode is A,and the total surface area of the opposing faces of the pair ofelectrodes is 2A. The volume enclosed between the faces is 2Ad for afixed spacing d between electrodes. For low-pressure operation, theelectrode spacing d must be increased for the reasons previouslydescribed. The plasma volume also increases with an increase in theelectrode spacing d, and therefore the surface area to volume ratiodecreases inversely proportional to increased spacing d. Thus, adecrease in operating pressure will result in the loss of some or all ofthe surface reaction advantages unless the surface area of theelectrodes can somehow be increased. Of course, one way to increase thesurface area of the electrodes is to increase the size of the reactorand hence the electrodes. However, for various reasons, including cost,as well as application or design constraints, this may not be desirableor even feasible. Therefore, it is necessary to find a way to furtherincrease the surface area of the electrodes within the reactor volumewithout increasing the size of the reactor for low pressureapplications, among other things.

The present invention addresses this problem by providing a new andunique electrode design. A primary objective of the new electrode designis to substantially increase the surface area of the electrodes withoutsubstantially increasing the volumetric size of the reactor. The newelectrode design provides highly efficient electrode surface reactionsover a significantly broadened range of operating parameters incapacitively coupled parallel plate plasma reactors and methods of thetype taught in the '633 patent without any significant increase in sizeof the reactor. As such, the new electrode design also greatly increasesthe range of applications for such reactors and methods.

SUMMARY OF THE INVENTION

The conventional thinking, as demonstrated in the '633 patent, has beento configure pairs of parallel plate electrodes with opposing facesextending laterally and without any intrusions into the open volumebetween the opposing or adjacent surfaces of the electrodes. It has beenthought undesirable to have any surface portions extending or intrudinginto the space between the electrodes because that would reduce thedistance between the electrodes at that point or those points. There hasbeen a significant fear that this would create a short circuit path thatwould cause arcing between the electrodes. Thus, it has been thoughtdesirable to design the electrodes so that their opposing surfacesshould be as flat as possible and as smoothly curved as possible toavoid this perceived problem. Moreover, for the reasons discussedpreviously with respect to the Paschen curves, there has been a fearthat reducing the spacing between electrodes could adversely impact thegeneration, maintenance and quality of the glow discharge plasma. Thishas been the conventional wisdom in the design and construction ofelectrodes.

The present invention contradicts the conventional wisdom relating toelectrode design and construction. In the present invention, a pair ofalternating polarity electrodes are configured with a plurality of“L-shaped” and “7-shaped” fin protrusions to form a so-called “L7”electrode structure. The protrusions extend into the open space betweenthe adjacent opposing electrodes and are arranged in interleavedfashion. The electrodes with their interleaved protrusions form roughlysquare “L7” shaped channels with one or more spaces or gaps, for exampleat one or more diagonal corners. A more expansive embodiment employs agrid type design wherein multiple pairs of opposite polarity electrodes,each having protrusions, are stacked together such that the protrusionsare interleaved in the space between the electrodes. The “L7” shape withthe interleaved protrusions maintains the electrode spacing d betweenthe parallel opposing faces of electrode pairs, but increases theelectrode surface area within a given volume by four times or more.Numerous protrusion/fin geometries are possible including continuouslycurved surfaces or “W” shaped surfaces, which can provide even moresurface area per unit of volume.

A plasma reactor/generator apparatus and method embodying the “L7”electrode design of the invention includes a reactor body with an openinterior volume, The reactor body includes a gas inlet and a gas outlet.The electrode apparatus is preferably configured as a unitary assemblythat can be inserted into and removed from the interior volume of thereactor as a unit. The electrode assembly is typically electricallyinsulated from the reactor body. In an open system, the electrodeassembly is enclosed within the interior of the reactor body and definesa plurality of subdivided gas flow paths between the gas inlet and gasoutlet. In a static or closed system, the electrode assembly subdividesor partitions the gas volume into a plurality of cells, which may beaccording to a desired pattern. A power source in electricalcommunication with the electrode assembly generates a voltage potentialbetween pairs of adjacent opposite polarity electrodes sufficient toignite and maintain a plasma in a selected gas to be processed in thereactor. In one aspect of the invention, a flow of gas is introducedinto the gas inlet at selected pressure, flow rate, and temperature andtraverses the subdivided flow paths adjacent the surfaces of theelectrodes to the gas outlet. A plasma having multiple distinct regions,preferably in at least partial communication, is generated and resultsin a highly efficient and complete chemical reaction of the gas upon thesurfaces of the electrodes, the reactor thus carrying out a selectedprocess on or employing the gas. In another aspect of the invention, theplasma generator is a closed system containing a gas. The plasma isformed in the gas, for example to generate luminescence. In this aspect,the invention is a plasma generator.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating Paschen curves for a number of typicalgases.

FIG. 2 is a graphical representation of a portion of a pair ofconventional capacitively coupled parallel plate electrodes of the typeused in conventional capacitively coupled parallel plate plasmareactors.

FIG. 3 is a graphical representation of a portion of a pair ofcapacitively coupled parallel plate electrodes embodying a preferred“L7” configuration according to the present invention.

FIG. 4 is an end view of the pair of “L7” electrodes shown in FIG. 3.

FIG. 5 is a side cutaway graphical representation of a stacked grid of“L7” electrode pairs with interleaved fins comprising a preferredembodiment of the present invention.

FIG. 6 is a plan view of a portion of a presently preferred embodimentof a first electrode forming an “L7” electrode pair with the secondelectrode of FIG. 7 for use in a stacked grid of “L7” electrode pairs asillustrated graphically in FIG. 5.

FIG. 7 is a plan view of a portion of a presently preferred embodimentof a second electrode forming an “L7” electrode pair with the firstelectrode of FIG. 6 for use in a stacked grid of “L7” electrode pairs asillustrated in FIG. 5.

FIG. 8 is a cutaway plan view showing a preferred “L7” electrode paircomprising the electrodes of FIGS. 6 and 7 overlaid.

FIG. 9 is a side elevation view of a preferred electrode assemblycomprising a stacked grid of “L7” electrode pairs as shown in FIGS. 6–8.

FIG. 10 is another side elevation view from a different elevation of thepreferred electrode assembly of FIG. 9.

FIG. 11 is a side elevation view of a preferred embodiment of acapacitively coupled parallel plate gas plasma reactor embodying thepresent invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

A description of the presently preferred embodiments of the inventionwill now be described with reference to the attached drawing.

FIG. 2 provides a graphical illustration of a conventional capacitivelycoupled parallel flat plate electrode pair 20 of the type commonly usedin almost all conventional plasma reactor designs today. Electrode pair20 comprises first plate electrode 22 and second plate electrode 24.First and second plate electrodes 22 and 24 each has first and secondsurfaces, each surface having an area A. The facing surfaces of firstand second electrodes 22 and 24 are separated or spaced apart by a fixeddistance d. The plate electrodes comprise the plates of a parallel platetype capacitor with each electrode electrically coupled to the oppositeterminal of a power supply 26, which may be either AC or DC type. Thus,at any given time, the electrodes 22 and 24 are of opposite polaritytype, such that a voltage potential (V) is present between them forigniting and maintaining a glow discharge plasma in a gas flowing in thespace between the electrodes. A simple calculation demonstrates that thetotal electrode surface area adjacent the open space between theelectrodes is 2A and the total volume of space between the twoelectrodes is Ad. Thus, the ratio of electrode surface area to volume is2/d cm-1 and, for a common electrode spacing of about 1 cm, the ratio ofsurface area to unit of volume is about two.

The surface reaction principle taught in the prior art '633 patent isbased on the knowledge that operating in a low pressure environment, astable glow discharge can be easily maintained. By maximizing theelectrode surface area per unit of plasma volume in the reactor,electrode surface reactions can be maximized. Large surface areaprovides a large reaction site for the gases to react on the surface.Absorbed and adsorbed gas molecules can easily find a site on thesurface and full population coverage ensures that when an ion orelectron hits the surface there is a high probability a chemicalreaction will be produced. Ignition and maintenance of the glowdischarge depends on the operating pressure and electrode spacing asdemonstrated by the Paschen curves of FIG. 1. The general rule of thumbis that for a parallel plate electrode, with the product of theanode-cathode spacing, d (cm), and the operating pressure P (Torr),i.e., Pd Torr-cm, at a value of about 1 Torr-cm, the minimum breakdownor plasma ignition voltage for most gases will occur at an electrodepotential of about 250–350 volts.

For operating pressures between about 500–1000 mTorr, for example, theoptimum spacing between the parallel plate electrodes would be about 1cm. If the operating pressure is increased above this range, the spacingd could be narrowed slightly to maintain the optimum breakdown or plasmaignition voltage range. However, if pressure is reduced below thisrange, the spacing d would have to be increased drastically to maintainthe optimum breakdown voltage. In other words, with changes in therequired operating pressure, it is necessary to change the spacing d tomaintain the operating Pd product as close to unity as possible if it isdesired to maintain the breakdown or ignition voltage as close to theminimum as possible. Otherwise, for low pressure operation the powersupply must be capable of providing very high voltages, well in excessof 1000V.

Thus, increasing the electrode spacing d is a typical approach used bythe industry for years. For example, the reactive ion etching (RIE)method used for plasma etching substrates, such as semiconductors,employs a capacitance coupled electrode and an operating pressure in therange of 10 to 100 mTorr. In commercial reactors, the electrode spacingd is in the order of 5–15 cm. This results in maintaining a relativelylow breakdown voltage as well as minimizing self-induced bias which canresult in undesirable radiation damage to the substrate being etchedfrom high-energy electron and ion bombardment.

The use of the larger electrode spacing d overcomes the difficultieswith initiating gas ionization and plasma ignition and maintenance asgoverned by the Paschen relationships. But the larger electrode spacinghas the undesirable effects of increasing the plasma volume, therebyrequiring more reactor space and more electrode surface area, which addssignificant expense. In order to construct an electrode having the samesurface area to operate at low pressure, the electrode spacing and hencethe volume must increase by multiples. However, if the reactor isdesigned with electrodes with larger surface area, it may be precludedfrom practical use in higher pressure ranges because the Pd product willbe on the increasing end of the high pressure side of the Paschen curve.

Today almost all of the functional parallel plate plasma reactors aredesigned based on the foregoing principles. For example, if theelectrode spacing d is about 1 cm it is relatively easy to generate andmaintain a glow discharge plasma at a pressure about 1 Torr for almostall useable gases. Similarly, if the spacing d were 2 cm, the optimumoperation pressure would be 0.5 Torr. The Paschen relationships alsosuggest that for a fixed electrode spacing d, the ability to generateand maintain a plasma will become slowly more difficult as operatingpressure increases, and that the minimum breakdown voltage will increaseslowly with rising pressure. Conversely, as operating pressuredecreases, the minimum breakdown voltage to initiate the plasmaincreases rapidly and it is much more difficult to generate and maintaina plasma The increase in the minimum breakdown voltage as operatingpressure increases is explained physically by the fact that at higheroperating pressures the main free paths of the gas molecules, atoms, andionized ions and electrodes in the space between the electrodes areshorter. Multiple collisions may therefore occur between excitedmolecules, atoms, ions, and electrons prior to the excited particlesreaching the electrodes. In each collision an excited particle losesenergy and transitions from a higher energy state to a lower energystate. Thus, over time fewer and fewer particles having a sufficientlyhigh energy state to initiate secondary ionization will be available andsecondary ion and electron generation can become localized and moredifficult. That condition will then require higher external operatingvoltage to generate and sustain the ionization breakdown process tomaintain the plasma.

With lower operating pressure, longer mean free paths reduce the numberof collisions of excited particles in the space between the electrodes.In that case, primary electrons that are accelerated between theelectrodes by energy acquired from the external applied voltage (AC orDC) have a higher chance of colliding with an electrode before collidingwith a gas molecule to ionize a neutral and produce more secondaryelectrons. More rapid loss of primary electrons combined with generationof fewer secondary electrons will require higher voltage from theexternal source to generate a stronger electric field and higher energyelectrons to ensure initiation of the ionization process and to maintainthe plasma. But higher field voltage accelerates the electrons fasterand shortens the time before they are lost to the electrodes, therebyreducing the probability for secondary electron production collisions.Consequently, as the operating pressure is reduced, the minimumbreakdown voltage required to initiate and maintain the plasma increasesrapidly.

In many of today's plasma reactor applications, for examplesemiconductor fabrication applications, the plasma reactor is requiredto operate at pressure ranges below 100 mTorr. In order to operate atsuch low pressures, the electrode spacing in such reactors must beincreased so that a plasma to be generated and maintained at areasonable voltage that is below about 1000V. Even then high voltageplasma generators are far more costly than low voltage generators.Furthermore, in order to dissipate the same power, P=IV, operation athigher voltage means lower current will be dissipated in the plasma.Since the chemical reactions involved require electron exchange, lowercurrent implies a lower chemical reaction rate. Therefore, to improvethe efficiency of the plasma reactor, it would be more preferable toemploy a lower voltage, higher current plasma.

At the same time, it is desirable to retain the successful features ofprior reactor designs as taught in the '633 patent, such as the parallelplate design, optimum electrode spacing, high surface area to volumeratio, long flow path and compact footprint.

FIGS. 3 and 4 provide a graphical illustration of the basic “L7”parallel plate electrode configuration, which comprises a basicpreferred embodiment of the present invention. As described below, the“L7” electrode arrangement overcomes the shortcomings of the prior art,described above, while retaining the successful features of the priordesigns as taught in the '633 patent.

The “L7” electrode arrangement comprises a first parallel plateelectrode 32 and a second parallel plate electrode 34 arranged inconventional opposing relationship. The electrodes 32 and 34 are eachelectrically connected to an opposite terminal of a power supply 36,which may be either an AC or DC type suitable for use in capacitivelycoupled parallel plate plasma reactors. Thus, the electrodes 32 and 34are of opposite polarity. Each electrode has a first surface 33 a, 35 aand a second surface 33 b, 35 b. In the preferred embodiment, the firstand second surfaces are formed integrally and at right angles, althoughvariations in both the construction and angle relationship areforeseeable. Surfaces 33 a and 35 a are spaced a fixed distance d fromeach other and their opposing faces are parallel to each other.Similarly, surfaces 33 b and 35 b are spaced a fixed distance d fromeach other and their opposing faces are parallel. Surfaces 33 b and 35 bprotrude or extend into the open space between the parallel surfaces 33a and 35 a such that the distal end of surface 33 b approaches surface35 a and the distal end of surface 35 b approaches the surface 33 a.Thus, the “L7” electrode pair arrangement described can form anapproximately square shaped long or subdivide a channel into individualcells. Preferably, the channels or cells are not completely enclosed.Small gaps are provided at one or more diagonal corners such that theelectrodes are spaced apart by a distance d′ between the distal end ofsurface 33 b and surface 35 a and the distal end of surface 35 b andsurface 33 a. Each of the surfaces 33 a, 33 b, 35 a, and 35 b has anarea of approximately A.

In this manner, the preferred “L7” electrode arrangement greatlyincreases the surface area to plasma volume ratio while retaining thesame electrode spacing d. For example, the total surface area presentedby the four surfaces to the square shaped channel or cell is 4A. Thetotal volume remains Ad as in the conventional parallel plate electrodearrangement of FIG. 2. Thus, the surface area to volume ratio of the“L7” electrode pair arrangement is approximately 4/d cm-1 or twice theratio of conventional parallel plate electrode pair arrangements.

The “L7” electrode pair arrangement contradicts conventional thinkingregarding parallel plate electrode pair design. Conventional thought isagainst bringing protrusions or extensions such as surfaces 33 b and 35a into the space between the parallel surfaces 33 a and 35 a, andcertainly not into close proximity with those surfaces. The conventionalthought has been that with the high volt potentials employed in manyreactor applications, arcing will occur between the adjacent electrodes.However, by reexamining the meaning of the Paschen curves of FIG. 1 withdeeper understanding, the inventor of the “L7” electrode design hasdetermined that in the low pressure ranges at which the “L7” electrodeis likely to find use, the space gap d′ between the opposite polarityelectrodes 32 and 34 at the corners can be made small enough so that thePd′ product will only permit a plasma to be generated and maintained inthe gap at a value of breakdown voltage higher than what the powersupply can provide. Thus, no plasma will exist in the gaps under thoseconditions. In addition, arcing and short circuit concerns are avoidedunder those operating conditions because the gap space d′ is too short adistance for electrons to accelerate sufficiently to cause ionization.An arc or short circuit can only occur when continuous ionizationcreates a conducting path (like a lightening path) between theelectrodes. These conditions cannot be physically present under the lowpressure operating regime in which the “L7” electrode is primarilyintended to be used. For example, a space gap of d′ =0.5 cm withelectrode spacing d=2 cm could allow the electrode to operate atpressure below 0.1 Torr with power supply voltage below 1000 V. Whenoperating pressure is increased to a relatively high pressure such as 2Torr, a suitable plasma can still be generated and maintained and willlikely include the gap regions, however, so long as the gap distance d′is maintained short enough, no arcing or shorting will occur across thegaps.

Another reason why the inventor has determined that arcing and shortingacross short gap distances d′ is not a concern is that the criticalrequirement for arcing is the presence of a point concentration ofdischarge such that a high current conducting path is created betweenthe point of discharge and the opposite electrode. In the “L7” electrodepair design, the electrodes form a conducting line such that there willbe a distributed discharge along the entire line (instead of a point)between them. Thus, sufficient voltage is not built up at any givenpoint on the electrodes under expected operating conditions to initiatea field emission wherein the electrons would have sufficiently highvoltage potential above the breakdown voltage potential to ionize theentire conduction path between the electrodes. In other words, becausethe plasma generated between the electrodes in the “L7” design is welldistributed along the entire opposing surfaces of the electrodes,insufficient potential is generated at any given point, including thepoints nearest the corners where the electrodes are in closestproximity, so that a complete conduction path between the electrodes canbe ionized and arcing occur. Thus, arcing and shorting are not a concerndespite conventional thinking to the contrary.

Another advantageous feature of the “L7” electrode arrangement is thatit intrinsically provides variable spacing between the preferredperpendicular surfaces of the electrode pairs. The range of effectivespacing extends from the gap space distance d′ to the distance d betweenopposite parallel surfaces of the electrodes, i.e., surfaces 33 a and 35a, or 33 b and 35 b. In fact there is an even larger distance ofseparation between the electrodes along the diagonal from the closed endcorner of each electrode to the closed end corner of the other, i.e.,from the point where surface 33 a meets surface 33 b to the point wheresurface 35 a meets surface 35 b. Thus, the new design intrinsicallyprovides variable spacing between the electrodes to accommodate optimumoperation at various pressures. The electrode design thus provides easyand efficient initiation and maintenance of a plasma over a broad rangeof operating conditions. Furthermore, as will be seen in more detailbelow, this design feature can be further extended. For example, with ahorn shape electrode design having an open end of smaller cross sectionthen the closed end further variable spacing range can be obtained,which will allow the plasma to select its optimal spacing under theoperating conditions to initiate the plasma easily.

The variable electrode spacing inherent in the “L7” design and itsextensions is a significant feature. Once a plasma is ignited, itbecomes a conducting sheet, itself acting as an electrode with manyconducting electrons present within it. Thus, the plasma itself is anadditional electron source to sustain the loss of electrons to theelectrodes. Thus, an easily ignited plasma is an easily maintainedplasma, meaning that the “L7” electrode design will permit easy andefficient initiation and maintenance of plasmas over a broad range ofoperating conditions.

FIG. 5 illustrates graphically the extension of the basic “L7” electrodepair design to a stacked electrode pair electrode assemblyconfiguration. Thus, FIG. 5 illustrates four electrodes 52, 54, 56, and58 stacked in a vertical configuration. Each electrode has two oppositesurfaces, thus electrode 52 has opposite surfaces 52 a and 52 b,electrode 54 has opposite surfaces 54 a and 54 b, electrode 56 hasopposite surfaces 56 a and 56 b, and electrode 58 has opposite surfaces58 a and 58 b. The electrodes are stacked such that their surfaces areall parallel to each other, thus surfaces 52 a, 52 b, 54 a, 54 b, 56 a,56 b, 58 a, and 58 b are all parallel to each other. Electrodes 52 and56 are connected in common to one terminal of a suitable AC or DC powersupply 60 and electrodes 54 and 58 are connected to the oppositeterminal of power supply 60. As a result the polarity of adjacentelectrodes in the stacked assembly alternate and each adjacent pair ofstacked electrodes forms an opposite polarity pair. Thus electrodes 52and 54 form one opposite polarity pair, electrodes 54 and 56 formanother pair and electrodes 56 and 58 form yet another pair. It isnotable that the opposing sides, e.g., 54 a and 54 b, of the electrodes,e.g., 54, are both used, which greatly increases the electrode surfacearea for chemical reactions to occur in the reactor. Consistent with thebasic “L7” design philosophy, each electrode has a plurality of “fins”or protrusions 64 extending at right angles from its opposite surfacesinto the space between adjacent electrodes. Thus, the fins 64 extendingfrom surface 52 b of electrode 52 and the fins 64 extending from theopposite surface 54 a of adjacent electrode 54 each extend into the openspace between the adjacent electrodes into proximity with the adjacentelectrode. As shown, for a variety of reasons it is preferred that thefins are located on adjacent electrodes of an electrode pair in analternating or interleaved arrangement. For one, this helps todistribute the plasma between the adjacent electrodes of each electrodepair which in turn helps ensure that no point ionization sources willresult in arcing or shorting between the electrodes. As will beexplained further below, it also helps to compartmentalize the plasma,resulting in a qualitatively better plasma and therefore improvedreaction efficiency. Still further, it ensures a long and circuitousflow path for the gas being treated in the reactor, which also improvesthe reaction efficiency.

This extension of the basic “L7” electrode design maintains theimprovement in surface area to plasma volume ratio with respect to priorparallel plate electrode configurations and multiplies it by stackingelectrode pairs in the reactor volume. Assume the distance betweenadjacent fins 64 of the same electrode, e.g., electrode 52 is d, and thedistance between opposing surfaces of adjacent electrodes in each pair,e.g., surface 52 b of electrode 52 and surface 54 a of electrode 54, isalso d. Assume also that the surface area between the adjacent fins oneach electrode is A and that the surface area of each fin is alsoapproximately A. Then in each “cell” bounded by adjacent fins of anelectrode, e.g., electrode 52, and the opposing parallel surfaces of theadjacent electrodes of each pair, e.g., surfaces 52 b and 54 a, thetotal electrode area is 4A. As in conventional parallel plate electrodedesigns, the electrode spacing is maintained as d, and therefore, theratio of electrode surface area to plasma volume seen by the plasma ineach cell or channel is approximately 4/d cm-1.

The “L7” design philosophy can be further extended to provide an evenhigher surface area to volume ratio by further subdividing theapproximately square shaped “L7” channels into cubic cells havingspacing of approximately d by adding additional fin elements aspartitions. This results in the plasma in each approximately cubicpartition seeing a surface to volume ratio of approximately 6/d cm-1.

Although the stacked “L7” electrode assembly of FIG. 5 is shown in crosssection, those skilled in the art will understand that the electrodes52–58 can be of various shapes. For example, as described below, each ofthe electrodes can be round. Similarly, although the fins 64 are shownin cross section, it will be understood that the fins can be of variousshapes including straight surfaces, curved surfaces, “U” shaped, “V”shaped, “W” shaped and horn shaped. Still further, the electrodes neednot be continuous surfaces, but may include one or more openings tofacilitate gas flow. Similarly, the fins 64 need not be continuoussurfaces, but may have openings for gas flow and plasma communicationpurposes.

Further, the dimensions and geometric shapes of the cells can be variedto modulate the plasma. For example, plasma intensity can be modulatedamong the cells according to a desired pattern. Areas of plasma focusand defocus can be generated. Plasma grids and pixels can also begenerated. Such modulation effects can be made periodic or according toother desired patterns.

Similarly to the basic “L7” design, the fins 64 of adjacent electrodes,e.g., electrodes 52 and 54 extend into proximity with the opposingsurfaces of the electrode pair, e.g., surfaces 52 b and 54 a, but remainseparated by a gap distance d′. For the same reasons discussed withrespect to the basic “L7” electrode design, if the gap distance d′ ismaintained small enough, then arcing and shorting between the electrodesis not a concern.

Referring to FIGS. 6–8, examples of preferred forms of fin elements,electrodes and electrode assemblies are shown. FIG. 6 is a plan view ofa portion of one electrode of an opposite polarity electrode pair, e.g.,electrode 52 of FIG. 5. The surface of electrode 52 shown in FIG. 6 issurface 52 b for example. Preferably electrode 52 is round in shape asshown in FIG. 8. Fins 64 are straight surfaces that extend outwardlyfrom the surface 52 b substantially perpendicularly. FIG. 7 is a topplan view of another preferred fin shape, namely an open ended “horn”shape. The horn shaped fin shown in FIG. 7 extends outwardly from thesurface 54 a of electrode 54 in FIG. 5 for example. As shown in FIG. 8,a plurality of such horn shaped fins 64 are located on the surface 54 aof electrode 54 for example so that the straight fins 64 extending fromthe surface 52 b of electrode 52 will be interleaved or interdigitatedbetween the sides of the horn fins 64 circumferentially around theentire surfaces of the adjacent electrodes 52 and 54. FIG. 8 thus showsvia a cutaway plan view how the straight fins 64 and the horn shapedfins 64 are interdigitated or interleaved when round electrodes, e.g.,electrodes 52 and 54, are stacked adjacent each other. Also as shown inFIG. 8, the preferred form of at least one of the electrodes of eachpair, i.e., electrode 52 in this example, has a central opening 80 forthe gas stream to flow through. Still further shown in FIG. 8 is thatthe diameter of the other electrode 54 will be slightly less than thatof electrode 52 to permit gas flow over the edge of the electrode andinto the next stacked electrode pair.

In addition to improving the electrode surface area to plasma volumeratio by inserting additional fin surface area into the space betweenthe electrodes, the fins also significantly increase the flow pathlength of the gas stream by converting the flat wide path between theelectrodes into multiple narrower paths. As mentioned previously, ifdesired the path can be even further subdivided into approximatelycube-shaped cells by interposing additional fin elements in the “L7”shaped channels. The partitioning of the wider path significantlyincreases the electrode surface area encountered by the gas stream as itflows through the reactor, without increasing the volume or size of thereactor.

This can be seen in FIGS. 9 and 10, which are two side elevation viewsof a stacked “L7” electrode assembly of the type illustrated partiallyin FIGS. 5–8, from different elevations. In FIGS. 9 and 10, it is seenthat the stacked electrode assembly comprises a series of alternatingstacked electrodes 92 and 94. As shown in FIG. 5, electrodes 92 areconnected in common to one terminal of a suitable power supply (notshown) and electrodes 94 are connected to the opposite terminal so thatadjacent stacked electrodes 92 and 94 are of opposite polarity. Eachelectrode 92 is disk shaped with a central opening 96 to permit gas toflow from layer to layer of the stack. Each electrode 94 is also diskshaped, but without a central opening. Preferably the diameter of theelectrodes 94 is slightly less than that of the electrodes 92 to permitgas flow over the outer edges of electrodes 94 from layer to layer ofthe stack. Obviously, electrodes 92 are electrically isolated fromelectrodes 94 via insulating spacers or the like. Electrodes 92 areformed with fins 64 that extend perpendicularly outwardly from eachsurface of each electrode 92 into proximity with but not into contactwith adjacent electrodes 94 on either side of each electrode 92.Similarly, each electrode 94 has “horn” shaped fins 64 that extendperpendicularly outwardly from each surface of each electrode 94 intoproximity with but not into contact with the surfaces of adjacentelectrodes 92 on either side of each electrode 94. Further, the straightand horn shaped fins 64 are preferably staggered so that they interleaveor interdigitate in the space between adjacent electrodes 92 and 94.With this configuration, the gas stream that enters the central opening96 of first electrode 92 must follow a plurality of smaller paths thatmeander between the interleaved fins in the space between firstelectrode 92 and first electrode 94, before flowing over the outer edgesof first electrode 94 to the next layer of the stack. There, the gasflows in multiple meandering paths between the interleaved fins in thespace between adjacent second electrodes 92 and 94 to the centralopening in the second electrode 92 and from there to the next layer ofthe stack, repeating the same meandering path through each layer of thestack until the last layer is traversed.

The electrodes 92 and 94 may be made of numerous suitable electricallyconductive materials that are known to those skilled in the art and thathave been conventionally used in plasma reactors in the past. Theelectrodes 92 and 94 illustrated in FIGS. 9 and 10 are preferably madeof stainless steel for low cost construction. Insulating materialsandwiching a conductive core can also be used when the power supply isa radio frequency type supply.

A further advantage of the preferred stacked “L7” electrode assemblyconfiguration of the invention is that the partitioning of the spacebetween the electrodes improves the quality of the plasma and hence thereaction efficiency of the reactor. The conceptual breakthrough is thata conventional parallel plate electrode pair may be viewed as two longparallel lines. Typically, the plasma is formed as a “sheet” in thecentral region of the space between the electrodes and has significant“blind spots” near the electrodes. The partitioning of the space betweenthe electrodes with fin elements breaks up the parallel plate blindspot. For this purpose, the fin elements that partition the spacebetween the electrodes could be two opposed plates having the samepotential as the anode or cathode electrode, two L-shaped and two7-shaped plates facing each other as “L7” facing plates. Thisarrangement of the “L7” shaped plates allows the plasma at the center ofthe channel to “see” essentially the entire space to all surfaces of theelectrodes even though the center of the channel is where the plasma isbeing generated and maintained. This is because the ions and electronsgenerated at the center of the channel radiate out to all enclosingsidewalks formed by the electrodes and partitioning fin elements of each“cell” or partition to cause chemical reaction. Reaction efficiency isthereby greatly improved.

Additionally, in extended parallel plate electrode reactors, the plasmais generated and maintained as a lateral sheet between the opposingsurfaces of the electrodes. This permits significant lateral chemicalinteraction in the plasma in the space between and parallel to theelectrodes. Since this chemical interaction occurs in the gas phase,they are likely to form molecular clusters and coagulated particlescalled “plasma dust” that can be entrained in the gas stream and exitthe reactor with the stream. This can cause serious problems, forexample with downstream pumps, particularly if the “plasma dust” happensto be caustic or corrosive in nature. By partitioning the plasma “sheet”into individual cells or segments, the present invention greatlyimproves the control over the lateral chemical interaction that resultsin formation of plasma dust. Indeed, the “L7” design permits relativelyeasy manipulation by designers of the length of the flow path and thenumber of partitions to provide improved control over the desiredsurface reaction vs. gas phase reaction balance.

Nevertheless, it is sometimes desirable to have some communication ofthe plasma in adjacent cells or segments. For example, due tomanufacturing tolerances or other reasons, there may be variations inthe dimensions of adjacent partitions or other factors that would resultin a weaker plasma being formed in one cell or partition than inadjacent cells or partitions. By permitting some communication betweenthe plasma in adjacent cells or partitions, an equalizing effect isachieved wherein the stronger plasmas in adjacent cells or partitionscan strengthen a weaker plasma in an adjacent cell or partition. Suchcommunication can be accomplished partially or totally via the use ofthe corner gaps as shown in FIG. 4 or the gaps between fins 64 andadjacent surfaces of electrodes 52–58 shown in FIG. 5. If additionalcommunication is desired, some or all of the fins may be provided withcommunication holes in their surfaces. The dimensions of the holes willof course depend on the application, the dimensions of the fins andelectrodes themselves, and the desired operating parameters.

Yet another advantage of the preferred “L7” electrode configuration ofthe present invention resides in material strength considerations.Formation and maintenance of a plasma generates substantial heat. And,the greater the surface area of the electrodes and the higher theoperating voltage, the more heat is generated. Thus, the electrodes aresubject to considerable thermal deformation stresses. Conventionalparallel plate electrodes comprising relatively large continuous sheetstend to accumulate large thermal stresses and are prone to structuraldeformation. Such deformation changes the electrode spacing and therebythe capacitance, electrical characteristics, and plasma properties. Insome severe cases, the structural deformation can result in shortcircuiting. Such considerations must be taken into account whenselecting the materials for the electrodes, etc. In contrast, the “L7”electrode structure, and particularly the stacked “L7” electrodeassembly structure, comprise numerous smaller surfaces andinterconnecting angled surfaces, which provide improved structuralsupport and stability compared to large flat sheets. Also since theplasma is partitioned into smaller components, the overall cumulativethermal stress on the structure is reduced. The improved intrinsicstructural strength allows the electrodes to be built with thinner sheetmetal than has been previously possible in conventional parallel platereactors designed for similar operating parameters and conditions. Thatin turn results in more space within the reactor interior volume to putin more surface area for a given compact volume, and to achieve evenbetter performance.

FIG. 10 is a side elevation of a capacitively coupled parallel plateelectrode plasma reactor embodying the preferred “L7” electrodeconfiguration of the present invention. The reactor 110 has a housingenclosing an interior volume (not shown) in which the electrode assemblyas shown in FIGS. 9 and 10 is mounted as a unit. The reactor housing maybe opened and closed using conventional means. If desired, the housingmay be provided with cooling surfaces for air cooling purposes. Aconventional gas inlet 115 is provided for receiving a gas stream to beprocessed. A gas outlet 120 is also provided for the processed gasstream to exit the reactor. External electrodes (not shown) forconnecting the terminals of a suitable power supply to the electrodesinternal to the reactor, for example as shown in FIG. 5, are alsoprovided.

FIGS. 10 and 11 illustrate a gas plasma reactor according to the presentinvention, which has been constructed. As constructed, the reactor has achamber defining a substantially cylindrical interior volume. Thereactor has an external height of approximately 420 mm and a diameter ofapproximately 290 mm. The interior is approximately 305 mm in height and254 mm in diameter. The chamber is constructed of aluminum and definesan interior volume of approximately 15,436 cm3. An electrode assembly,as shown in FIG. 10, made of 316L stainless steel comprises 6disk-shaped anode-cathode pairs. The spacing between adjacent anodes andcathodes is approximately 1 inch near the gas inlet of the reactor andis decreased slightly nearer the gas outlet of the reactor to promotegas processing efficiency as gas flows through the electrode assemblybetween the inlet and the outlet. The electrode assembly has an outerdimension slightly less than 254 mm and is approximately 300 mm tall.The anode disks are provided with center holes and the cathode diskshave an outer dimension slightly less than the anode disks to provide ameandering gas flow path between adjacent anode-cathode pairs. Sixteenhorn shaped fin elements are equally spaced around the circumference ofeach surface of each cathode and sixteen plate shaped fin elements areequally spaced around the circumference of each surface of each anodewith the plate shaped fins being interleaved between each horn shapedfin and between each leg of each horn shaped fin (see FIG. 8). The hornshaped fins and plate shaped fins are spaced and dimensioned to defineapproximately one cubic inch segments or subdivisions of the flow pathbetween adjacent anodes and cathodes. The total electrode area withinthe interior volume of the reactor is thus approximately 27,700 cm2 andthe ratio of electrode surface area to volume is approximately 1.8.

Tests to initiate and maintain a plasma in air have been conducted usingthe foregoing reactor over a range of operating pressures and voltages.These tests have been conducted employing an Advanced Energy IndustriesModel 2500E power supply, modified to operate at 100 Khz. As-modified,the power supply is load rated at approximately 1500W. As tested, thereactor has successfully initiated and maintained a plasma in air atpressures up to 500 Torr at approximately 1000V with a load impedance ofapproximately 100 ohms and at pressures down to about 18 mTorr atapproximately 1400V with a load impedance of approximately 1000 ohms.

The foregoing descriptions of the presently preferred embodiments of theinvention are intended to be exemplary in nature rather than limiting.Various changes and modifications to the preferred embodiments will beapparent to readers skilled in the art and may be made without departingfrom the spirit of the invention. For example, various discloseddimensions may be changed and different materials substituted for thosedisclosed. Different geometric shapes may be selected for theelectrodes, the reactor chamber, the fin elements and the like, forexample “U,” “V,” “W” shapes, or even cylindrical, spherical or conicalshapes. Thus, the present invention virtually eliminates previousconstraints and restrictions on electrode geometries and designs.Operating parameters may also be altered. The scope of the invention isintended to be defined and limited not by the specific details of thepreferred embodiments, but by the appended claims.

1. A gas plasma reactor comprising: a reactor chamber defining aninterior volume and having a gas inlet and a gas outlet; an electrodeassembly positioned in said interior volume in communication with saidgas inlet and said gas outlet, said electrode assembly adapted to beinterposed in a flow of gas from said gas inlet to said gas outlet, saidelectrode assembly comprising: a plurality of anodes and cathodes havinga plurality of anode and cathode surfaces, wherein the anode and cathodesurfaces face each other and are spaced apart from each other, defininga meandering flow path for a gas; said anodes having a common electricalconnector and said cathodes having a common electrical connector, and aplurality of electrically conductive fin elements connected to facingsurfaces of said anodes and cathodes and interposed in the space therebetween thereby substantially partitioning said meandering flow path;and an electrical connector for connecting a source of electrical powerto said electrode assembly.
 2. The gas plasma reactor of claim 1 whereinsaid plurality of facing anode and cathode surfaces provide a combinedelectrode surface area that facilitates gas reaction over the course ofsaid flow path, and wherein said fin elements are arranged to have aplurality of facing, spaced apart surfaces, thereby increasing saidelectrode surface area to further facilitate said gas reaction.
 3. Thegas plasma reactor of claim 2 wherein the fin elements connected to thecathode are interleaved with the fin elements connected to the anode. 4.The gas plasma reactor of claim 3 wherein at least some of saidplurality of fin elements are shaped approximately as a “V”, “V” or “W”.5. The gas plasma reactor of claim 4 wherein said plurality of facingsurfaces of said anodes and said cathodes are substantially disk shapedand wherein said reactor chamber defines an interior volume that issubstantially cylindrical.
 6. The gas plasma reactor of claim 5 whereinat least one of said plurality of anodes and said plurality of cathodeshas an opening in a said facing surface to facilitate gas flow betweenan adjacent anode and cathode.
 7. The gas plasma reactor of claim 6wherein at least one of said plurality of anodes and said plurality ofcathodes has a diameter that is smaller than the diameter of the otherto facilitate gas flow between adjacent anodes and cathodes.
 8. The gasplasma reactor of claim 1 wherein said plurality of fin elements extendoutwardly from said plurality of anodes and said plurality of cathodesrespectively at an angle.
 9. A gas plasma generator comprising: achamber defining an interior volume; an anode and a cathode positionedin said interior volume and adapted to be interposed in a volume of gas,said anode and cathode each having a plurality of surfaces, said anodeand cathode surfaces being interleaved with adjacent surfaces facingeach other and spaced apart to define a volume for said gas; a commonelectrical connector for each of said plurality of surfaces of saidanode, and a common electrical connector for each of said plurality ofsurfaces of said cathode; and a plurality of electrically conductive finelements connected to facing surfaces of said plurality of surfaces andinterposed in said volume between said facing surfaces of said anode andsaid cathode thereby substantially partitioning said volume.
 10. The gasplasma generator of claim 9 wherein said plurality of fin elements arearranged to have a plurality of facing, spaced apart surfaces, therebyincreasing the electrode surface area in the volume between said facingsurfaces of said anode and said cathode.
 11. The gas plasma generator ofclaim 10 wherein said plurality of fin elements connected to said anodeis interleaved with said plurality of fin elements connected to saidcathode.
 12. The gas plasma generator of claim 9 wherein said pluralityof fin elements arc shaped approximately as a “U,” “V,” or “W.”
 13. Thegas plasma generator of claim 9 wherein said facing surfaces of saidanode and cathode are substantially disk shaped.
 14. The gas plasmagenerator of claim 13 wherein said fin elements are dispersedsubstantially circumferentially around said facing surfaces of saidanode and cathode.
 15. The gas plasma generator of claim 13 wherein atleast some of said plurality of facing surfaces of said anode and saidcathode have an opening to facilitate communication of gas betweenadjacent surfaces.
 16. The gas plasma generator of claim 15 wherein atleast some of said facing surfaces of said anode and said cathode have adiameter that is smaller than the diameter of an adjacent surface tofacilitate communication of gas between adjacent surfaces.
 17. The gasplasma generator of claim 9 wherein said plurality of fin elementsextend outwardly from said plurality of facing surfaces of said anodeand cathode at an angle.